The present invention relates to an ultra high carbon steel composition. It is known that conventional steel has a coarse grain size on the order of 50-100 microns. It is also known that steel having a very fine grained iron matrix is characterized by superplastic flow at elevated temperatures. However, fine grained iron tends to be unstable and grow at elevated temperatures. Thus, stabilization of the small grain size at such temperatures is necessary in order to prevent the destruction of such exceptional plasticity.
One attempt at stabilizing fine grained iron structure is set forth in an article by Morrison, entitled "Superplastic Behavior of Low-Alloy Steels", Transactions, ASM, Vol. 61, 1968, 423. That paper suggests the addition of manganese or phosphorus to the steel to increase its plasticity at elevated temperatures by stabilizing the fine grains. There are many disadvantages to this approach. The paper recites a temperature range of 850.degree. .+-. 25.degree.C for the desired plastic flow, which is too narrow a range for working temperatures on an industrial scale. Furthermore, phosphorus and manganese are relatively expensive. Another deficiency of the Morrison technique is its disclosure of the further addition of relatively expensive aluminum and vanadium to retain the fine grain size. The latter element is very expensive. Finally, the steels disclosed in the Morrison paper in Table 6 at page 433 have relatively poor cold temperature properties.
The possibility of superplastic behavior of steels similar to those of the Morrison article is disclosed in a paper by Schadler entitled "The Stress-Strain Rate Behavior of a Manganese Steel in a Temperature Range of the Ferrite-Austenite Transformation", Transactions, AIME, Vol. 242, 1968, 1281. Schadler found superplastic behavior with iron containing 1.9 weight percent manganese in the temperature range where ferrite and austenite phases coexist. He concluded that superplasticity could only be achieved at commercially unattractive strain rates (e.g., 0.1 %/minute). A further disadvantage, set forth with respect to the Morrison publication, is the narrow temperature range over which superplasticity can be expected to exist. Another commercial problem is the requirement for the addition of relatively expensive manganese.
Another approach is illustrated in a paper by Marder, entitled "The Effect of Carbon Content, Test Temperature, and Strain Rate on the Strain-Rate Sensitivity of Fe-C Alloys", Transactions of the Metallurgical Society of AIME, Vol. 245, June, 1969, 1337. There, the properties of iron-carbon alloys of high purity were studied in a composition range from 0.2 to 1.0 % carbon. The maximum elongation for the 0.8 % carbon content is 98 %. The article expresses concern with void formation at the boundary between the iron-cementite interface causing premature failure during deformation. It also suggests that cementite is brittle at warm temperatures. FIG. 5 of the paper illustrates a decrease in the strain rate sensitivity exponent, m, when the carbon content is increased from 0.8 % to 1.0 % carbon. The paper suggests the reason for this decrease is that the ferrite grains are no longer equiaxed at the higher carbon content. Thus, the paper teaches away from further increasing the carbon content.
Another attempt at a superplastic steel is set forth in a paper by Yoder et al., entitled "Superplasticity in Eutectoid Steel", Metallurgical Transactions, Vol. 3, March 1972, 675. There a worked commercial eutectoid steel was found to exhibit good ductility in a temperature range (710.degree.-720.degree.C) which is too narrow for use in industrial forming operations. This indicates a grain growth above that temperature range but does not offer any suggestions on a technique for expanding this range. Another deficiency of this reference is that maximum elongation is 133 %, far below superplastic behavior. Furthermore, there is no disclosure of room temperature strength.
A steel which is capable of very large deformation over a wide range of temperatures during fabrication to large strains without cracking and under all externally applied forces for minimum expenditure of energy is desirable. Furthermore, such steel should be characterized as strong, tough and possessing of high ductility for final use. A third important feature which is desirable in steel is of course that it be inexpensive. Ultra high carbon steels, i.e., with a carbon content in excess of 1.0 %, have not been considered capable of accomplishing all of these criteria. This is perhaps because they are normally considered as potentially too brittle for ambient temperature application. Furthermore, their high temperature characteristics have apparently not been explored.